In particular liquid filtering arts it is necessary to prevent dirty liquid in a filter cartridge from passing out of the filter cartridge and into the immediate area around the filter, i.e., the filter housing when the filter is withdrawn. This is true in the situation where liquid is pumped through the cartridge, becomes clean or filtered, and drops into a tank or trough below the cartridge. When the cartridge needs to be replaced and dirty liquid remains in the cartridge after pump shutoff, such liquid must remain in the cartridge. Otherwise, the dirty liquid can leak out and contaminate the clean liquid in the tank.
The process of wire electro discharge machining (wire EDM) uses a filtration system as described above. A wire EDM system basically cuts a part by emitting electronic charges which erode the surface of the part being machined.
A non-conducting liquid is used to flush and cool the part while the part is being cut with the wire EDM system. Since the liquid is in a closed system, i.e. the liquid is reused, the liquid must be filtered to remove the particles it flushed away from the part being machined. Otherwise the particles may clog the machine or render the non-conducting liquid a conducting liquid. The filter cartridge for filtering such liquid generally requires replacing about once every 100 to 150 hours of process time.
The current filter cartridge has a perforated cylindrical housing, a central tube for conducting liquid into the housing from the bottom of the housing, and a filter media disposed between the tube and the housing. In operation, liquid is pumped through the tube into the housing. The liquid passes through perforation all along the length of the tube into the housing. It passes then through the filter media and out through the perforation in the housing, where it falls into a tank before being recirculated. Unfortunately, when the filter cartridge is removed, any residual liquid in the filter can pass back through the holes in the central tube and out the cartridge--and into the tank with the clean liquid. This requires that the tank be cleaned after each time the filter cartridge is replaced.
A filter cartridge disclosed in the U.S. Pat. No. 1,805,450 to Harvey has a central tube which is open only at the top. This helps trap dirty liquid in the filter and prevent such liquid from leaving the filter except through the filter media as clean liquid. Unfortunately, the Harvey filter has filter media contiguous with or abutting the central tube. In other words, there is no space between the tube and the filter media in which the liquid may collect before passing through the filter media. The liquid becomes filtered as soon as it leaves the tube. This arrangement causes the filter media closest to the opening on the tube to become used up, even clogged, before other areas of the filter media. This is not an efficient use of the filter media.